1. Field of the Invention
This invention relates to a hydraulic power steering apparatus suitable for use in automobiles and the like.
2. Discussion of Related Art
A conventional hydraulic power steering apparatus is composed of a pump 1, a reservoir 2, a power cylinder 3 for generating an assisting force, and a rotary valve 4, as shown in FIG. 1. The rotary valve 4 is composed of a pair of bridge circuits having variable orifices 4A-4D and 4E-4H, respectively. When the steering wheel 6 is rotated, the rotary valve 4 is operated so that the pressurized fluid output from the pump 1 is supplied to one of the chambers of the power cylinder 3, while fluid is drained from the other of the chambers to the reservoir 2. Although the power cylinder 3 is illustrated in the drawing as being arranged in each of the bridge circuits C1 and C2, the cylinder 3 is a single cylinder.
In such power steering apparatus, the rotary valve 4 is desired to have such a characteristic that the differential pressure in the power cylinder 3 is reduced to zero at rotational angle positions near the neutral position (rotational angle positions smaller than a predetermined angle position .theta.1), whereby the generation of the assisting force is stopped. With this characteristic, it becomes possible to give a driver a steady steering feeling similar to that in a non-assisted power steering apparatus in the vicinity of the neutral position.
However, the conventional rotary valve does not have the above-mentioned desired characteristic. Namely, in the conventional power steering apparatus, the opening areas of the variable orifices 4A-4H immediately start to increase in response to the rotation of the steering wheel 6, even when the steering wheel 6 is located near the neutral position. Accordingly, the differential pressure in the power cylinder 3 immediately starts to increase and gradually increases even more as the rotational angle increases.
Since the rotational angle at which the power assistance is started is not unclear, the conventional power steering apparatus gives the driver no steady steering feeling but instead a vague feeling in the vicinity of the neutral position.